Oil shale is a sedimentary geologic formation generally found in the western states of the United States. The oil shale contains a hydrocarbonaceous component called kerogen. By mining the oil shale and heating it in a retort, the kerogen component is liquified and can be recovered for refining into an oil product similar to petroleum oil products. The retorting of oil shale can be conducted in a surface retort vessel which is fed with traditionally mined, particulate oil shale, or in in-situ retorts wherein the oil shale is blasted into a concise rubble pattern within its geologic formation, in which the formation forms the retort itself. The particulate or rubble oil shale is then ignited by the combustion of a gas forced through the retort. A portion of the oil shale is burned during the heating operation in which that amount of the kerogen is also lost. However, as the unsteady state, batch retorting process progresses, the hot combustion product gases pass down through the shale, heating it and retorting it to drive out gas and oil vapor products that are carried out with the combustion products and cooled. The retorted shale contains residual carbon that sustains the burn as retorting progresses downward. This heat and combustion process requires a sustained flow of oxygen gas but only an initial flow of fuel gas. With air as the typical oxidant, a diluent is required to moderate the peak combustion temperatures to avoid melting the shale to a slag and to avoid producing excessive energy consuming carbonate decomposition. Steam and carbon dioxide are known retort diluent gases. As a retort operation proceeds, substantial quantities of liquid hydrocarbon oil and off-gases are produced. The off-gases consisting of combustion products, oil shale volatiles and diluent gas are separated from the liquid phase and cleaned and vented or can be recycled. Various sulfur compounds such as hydrogen sulfide and carbonyl sulfide are found in the off-gases and are a problem to the proper disposal or use of the off-gases. It has been found that the sulfides can be absorbed onto the spent oil shale if the off-gas is recycled. Additionally, the recycled off-gas, when depleted of any BTU fuel components, can serve as an excellent moderator or diluent gas for combining with the oxidant feed, such as oxygen, to the retort as the combustion and heat inducing media.
In surface and in-situ combustion type coal gasification processes, moderators are typically added to the input air or oxygen. In surface gasifier retorts, steam has typically been used to hold peak temperature to levels where the ash will not slag. In in-situ coal gasification processes, steam has been added to avoid excessive temperatures with high heat losses into surrounding strata and to avoid burnout of the oxidant injection lance. Steam has the advantage that it is easily separated as condensate by cooling the gasifier effluent. It has the disadvantage that the condensate requires expensive treatment to remove contaminants and that energy requirements for steam generation are high. In the established Lurgi dry ash moving bed gasifier retort using steam and oxygen, the energy required for the steam is 3 to 4 times greater than required to supply the oxygen. Carbon dioxide has been proposed as a combustion moderator for coal gasification, but has not been widely used even though it has been potentially available for recycle from the gasifier effluent. High energy requirements of existing processes for separating the CO.sub.2 for recycle have presumably discouraged its use.
In both methods, coal gasification and oil shale retorting, it is environmentally as well as economicaly beneficial to recycle the carbon dioxide off-gases as a diluent gas for the retort operation and to absorb any sulfur containing components from the off-gases onto the remaining combusted media, i.e. spent oil shale or coal ash by way of the separated and recycled diluent gas stream. This method avoids the costly preparation of steam diluent and provides greater selectivity than air mixture diluent, while at the same time taking advantage of the use of the remaining media to rid the process and the atmospheric environment of noxious sulfur contaminants such as sulfides in various forms.
Various prior art processes have been developed for the refinement and the recycling of the off-gas products of coal gasification and oil shale retorting, as generally described above, especially in-situ oil shale retorting. These prior art processes generally suffer from high energy consumption and a complexity of process apparatus which requires a high capital expenditure.
In U.S. Pat. No. 2,886,405, a process is disclosed for the separation of carbon dioxide and hydrogen sulfide from gas mixtures utilizing a chemical absorbent solvent, such as hot potassium carbonate. As is typical in chemical solvents, the enriched solvent is regenerated by a boiling and steam stripping operation. Such a regeneration is an energy intensive operation.
The prior art in U.S. Pat. No. 4,014,575 teaches that off-gases from oil shale retorting can be recycled through spent oil shale beds for the deposition of sulfur compounds from the off-gas onto the particles of the oil shale bed. This can be done in conjunction with the water scrubbing of the off-gases in a Venturi scrubber.
Another method has been utilized to scrub the off-gases from oil shale retorting wherein water containing basic components from an oil shale retort bed is contacted with the acid gas containing off-gas stream of an operating oil shale retort. The basic pH water neutralizes the acid off-gases and the latter can be recycled for retorting or burned if sufficient BTU energy can be derived. This process is described in U.S. Pat. No. 4,117,886.
In U.S. Pat. No. 4,158,467, a process for the recycling of oil shale retort off-gases is disclosed wherein the hot potassium carbonate solvent of U.S. Pat. No. 2,886,405 mentioned above, is utilized. As stated before, the utilization of chemical absorbent solvents in such an operation is energy intensive due to the complexity of regenerating such solvents for reuse. Additionally, the chemical absorption process is essentially non-selective, i.e. complete absorption of acidic sulfur compounds would be accompanied by complete absorption of contained CO.sub.2.
The removal of acid gas components from gas streams is discussed in U.S. Pat. No. 4,169,133 wherein the carbon dioxide acid gas component is frozen out of the main gas stream. A process wherein a solid product is produced from a gas clean-up operation is not conducive to the recycling of such a component, such as in the present invention.
In U.S. Pat. No. 4,169,506, the scrubbing of off-gases from in-situ retorting of oil shale is set forth. The scrubbing utilizes caustic soda in conjunction with a deoiling process. In this instance, the scrubbed sulfur components are passed to a Claus plant for refinement to elemental sulfur.
In South African Published Application Ser. No. 77/7157 of Dec. 1, 1977 a process is disclosed for the separate removal of sulfides and carbon dioxide from a coal gasification gas stream. Externally supplied refrigeration is necessary to operate a complex solid/liquid absorbent stream in a process which operates on carbon dioxide containing streams in the 55% carbon dioxide range. Corresponding U.S. Pat. No. 4,270,937 of June 2, 1981 discloses similar subject matter.
The attempts by the prior art to solve the problems of economical provision of a diluent gas for the injected oxidant and handling of significant quantities of off-gas generated in oil shale retorting and coal gasification, whether these operations are undertaken in-situ or in external surface retorts are deficient for a number of reasons, including: the energy intensive nature of their scrubbing recovery operations, the necessity for regeneration of chemical solvents by steam stripping operations and the need for large quantities of water for scrubbing operations in retorting locations which may be deficient in adequate water resources to make such recovery systems operational.
The present invention overcomes these obstacles by providing a low energy, low temperature or cryogenic system for the recovery of recyclable gases from the off-gases of carbonaceous combustion retorting, such as oil shale and coal gasification retorting. The present invention achieves this recovery of recyclable gases such as carbon dioxide and acidic sulfide gases, either by cryogenics (low temperature) individually, or cryogenics and physical absorbent solvents used in conjunction with one another. The physical absorbent solvents are regenerated in a low energy process as compared to the chemical absorbent solvents of the prior art. Furthermore, the present invention process does not require the utilization of potentially scarce and valuable water resources at the site of the retorting operation.
The carbon dioxide separation and recycle of this invention is also useful in carbon dioxide miscible flood enhanced oil recovery operations. In this type of operation, carbon dioxide under high pressure is injected into an injection well to pressurize and lower the viscosity of oil formations which require pressure maintenance or secondary recovery in order to achieve economic production. High pressure carbon dioxide brings oil into solution and pushes oil toward the production well. As pressure is reduced at the surface of the production well, oil is separated as a liquid phase from carbon dioxide and oil derived contaminants in a gas phase. This gas phase can be introduced into the process of the present invention.
In oxygen fireflooding, a tertiary form of enhanced oil recovery, an oxidant such as air or preferably oxygen is injected into an oil formation and combusted either spontaneously or by an ignition media. The combustion heats the subterranean oil to volatilize a portion thereof and coke the remaining portion. The coked portion sustains the burn in conjunction with the oxidant necessary to heat the oil formation for successful tertiary production. This subterranean combustion produces significant levels of carbon dioxide which can be processed by the process of the present invention and sent to other carbon dioxide utilizing processes, such as the former systems mentioned above.